AFAK, any knife that can be potentially used as a weapon is prohibited.
Whether a knife is a potential weapon is vaguely evaluated based on several standards: knife type, blade length, and the angle of the tip.
In Beijing, any dagger that is longer than 100mm, triple-bladed dirk, and any spring-loaded knife that locks open are prohibited. Any other knives that have a blade length over 120mm and a tip angle less than 60 degree are also prohibited.
If in some rare cases a policeman want to confiscate your knife, make sure to ask for a receipt. I've read about stories about policemen attempting to confiscate SAKs, and they do that just because they want those knives. So insisting on having a legimit receipt usually stops such attempts.
Finally, try to make some Chinese friends and ask about their advice before you plan to visit somewhere, buy something, or eat something. That will help you avoid a lot of nasty stuff or people.
good luck!